1. Field of the Invention
The invention relates to a switching power supply and, in particular, to a switching power supply having increased power conversion efficiency at light load.
2. Description of Related Art
Since the line voltage is AC power while the operating power for computer devices is DC power, a power supply for converting the AC power to DC power is required for most computers.
With reference to FIG. 6, a conventional power supply includes an AC to DC converting unit 81 and a DC to DC converter 82. The AC to DC converter 81 electrically connects to the line voltage to obtain the AC power and converts the AC power into a DC power. The DC to DC converter 82 electrically connects to the AC to DC converter to receive the outputted DC power and further converts the received DC power into a DC power for supplying to a load 90.
Currently, a commonly seen AC to DC converting unit 81 includes a rectifier and a boost power factor correction (PFC) circuit. In addition to converting the AC power into DC power in phase with the AC power, the AC to DC converting unit 81 also boosts the voltage level of the DC power. For example, the DC power is boosted to 380 V. The DC to DC converter 82 uses a voltage bucking circuit correspondingly so as to reduce the DC power to a desired voltage level being usable by the load 90. The DC to DC converter 82 can be a flyback converter, a forward converter, or a push-pull converter. The overall power converting efficiency of the DC converter 82 is determined by on and off operations of a power transistor in the DC converter 82.
Since the voltage imposed on the source and drain of the power transistor is equal to the DC power voltage output from the AC to DC converting unit 81, the imposed voltage on and the current flowing through the source and drain result in switching loss when the power transistor is switched on and off. If the load 90 is a medium or heavy load, the overall power to the load 90 is relatively large. In other words, the proportion of the switching loss to the output power is relatively small. The power converting efficiency is acceptable.
However, if the load 90 is a light load, the proportion of the switching loss and other consumption for maintaining basic circuit operations to the output power is obviously higher because the output power is small. As a result, the overall operating efficiency of the power supply drops down. Therefore, it is necessary to find a solution for this problem.